Numerous fields of technology require that thin layers are applied on substrates. For example, glass panes are coated in order to provide them with special properties or watch cases comprising a less noble material are coated with a layer comprising a noble material.
Numerous processes for the application of thin layers on substrates have already been suggested, of which only electroplating and plasma coating will be mentioned. Plasma coating has gained increasing importance because this process permits a multiplicity of materials as coating material.
In order to generate a plasma suitable for coating, various processes have also been suggested. Of great interest among these processes is the cathode sputtering process because of its high coating rates. The coating rates can still be increased further if microwaves are irradiated into the volume in front of the sputtered cathode.
Several devices for microwave-enhanced sputtering are already known (U.S. Pat. Nos. 4,610,770 and 4,721,553; and DE-A-3 920 834). In these devices the microwaves are introduced into the plasma region either parallel or perpendicularly or at a given angle to the substrate surface. Through the cooperation of the magnetic field of the magnetron with the microwaves an electron cyclotron resonance (ECR) effect can be established, which increases the ionization of the plasma particles.
Of disadvantage in most of the known devices is, however, that the ECR condition occurs in the proximity of the sputter cathode where excitation of the particles is not as necessary as in the region of the substrate. In order to remedy this disadvantage in the device according to DE-A-3 920 834, the microwaves are radiated onto a substrate which, in turn, is penetrated by the field lines of a permanent magnet. Hereby the ECR condition is fulfilled directly above the substrate. However, in this known device, a plasma discharge can only occur up to a given minimum voltage and up to a given minimum pressure. Even with very strong magnetic fields on the target surface, the plasma ignition voltage cannot be lowered further. High discharge voltages, however, lead to undesirable effects, such as the inclusion of noble gases in the substrate structure, or to radiation damages.
In order to further lower the plasma ignition voltage, a waveguide with which microwave energy is guided to a target surface, is positioned within the vacuum chamber at the lateral edge of the target. The ionization of the plasma is hereby considerably increased so that it becomes possible to sputter at a low discharge voltage and at high current. Discharge voltages of below 100 volts are realizable.